Field of the Invention
This invention generally relates to vibrational piezoelectric energy harvesting devices where the mechanical vibrations are transformed to electrical energy to power electronic apparatus. More specifically, the present invention relates to multilayered piezoelectric vibrating mechanical devices including a composite shim acting as a flexural mechanical spring.
Description of the Related Art
Harvesting vibrational energy using resonant piezoelectric structures has long been investigated and disclosed. One architecture is a beam structure where an amorphous shim, also called “mechanical spring,” is sandwiched between two piezoelectric layers, each having an electrode plated on a main surface. When such a bimorph structure is mechanically bent, flexural stresses are created in the piezoelectric layers and electrical charges proportional to the amplitude of bending are produced on the electrodes. Shunting the electrodes through an electronic circuit allows harvesting of the energy produced. Usually such piezoelectric harvesters operate at a resonance frequency of the vibrational structure in order to maximize the deformation of the piezoelectric material and therefor the electric energy output.
Common piezoelectric flexural energy harvesters operate in a 33-conversion mode (polarization is collinearly oriented compared to relevant stress) or under a 31-conversion mode (polarization direction is orthogonal to relevant stress), have parallelpiped shapes, and are of a uniform design. A common architecture of such harvesting devices can be represented by a cantilever (i.e., one-end clamped) beam structure having a loading mass mounted at the free end and free to vibration in a privileged direction. This structure is usually optimized for operations at a desired resonant frequency and to increase the flexural stress of the beam in order to maximize the energy output.
However, such architecture is subjected to performance limitation since the flexural stress over the device area does not remain constant. Indeed, typical piezoelectric harvesting cantilever beams are mechanically clamped at one end and loaded with a seismic mass at the other end for optimal operations. Therefore, the clamping area of the beam is intensively stressed during vibrations and provides the primary contribution to electrical energy production. The opposite end, where the seismic mass is generally mounted, will contribute in a much lower proportion because of the much lower locally applied stress. Thus, the inherent solution to increase the electrical power of device at constant vibration consists in maximizing the flexural stress level at the clamping area at the expense of the device reliability and durability.
A high level of flexural stress means that the harvesting device can locally (close to the clamping area) reach very high depoling voltages and critical tensile depoling stress as well. In other words, maximizing output performance will inherently degrade the product lifetime and reliability. The mandatory trade-offs between performances and lifetime for all conventional energy-harvesting designs represent the strongest limitation of current available products.
To achieve an improved uniform stress distribution, several strategies have been disclosed and experienced in the prior art. Most of them implement geometric designs to fine tune the flexural stress distribution along the length dimension, either by tapering the thickness (US20100194240A1, WO2012107327A1) or by tapering the lateral dimensions (U.S. Pat. No. 7,948,153B1). In these references, the flexural stress distribution is “geometrically” controlled either by reinforcing the structure with oriented stiffeners or by changing the thickness of the bimorph structure along the beam length or width dimension, i.e. by tapering the structure thickness. However, the above references require intricate realization processes that affect the reliability and the manufacturing costs of the subject devices.